The US and Europe have dramatically increased the pressure on the Syrian president, Bashar al-Assad, with Barack Obama leading a demand by world leaders for him to surrender power.

Obama declared the "sustained onslaught" of Assad's regime against pro-democracy protesters had cost it all legitimacy. The US president was joined by David Cameron, French president Nicolas Sarkozy and German chancellor Angela Merkel, as well as the EU in demanding Assad immediately resign.

Obama said the Syrian people's pursuit of democracy was an inspiration that had been met with "ferocious brutality" by their government.

"The future of Syria must be determined by its people, but President Bashar al-Assad is standing in their way. His calls for dialogue and reform have rung hollow while he is imprisoning, torturing and slaughtering his own people," Obama said. "We have consistently said that President Assad must lead a democratic transition or get out of the way. He has not led. For the sake of the Syrian people, the time has come for President Assad to step aside."

Cameron issued a joint statement with Sarkozy and Merkel that noted Assad had ignored appeals from other Middle East states, the Arab League and the United Nations security council to end the violent crackdown:

"Our three countries believe that President Assad, who is resorting to brutal military force against his own people and who is responsible for the situation, has lost all legitimacy and can no longer claim to lead the country. We call on him to face the reality of the complete rejection of his regime by the Syrian people and to step aside in the best interests of Syria and the unity of its people."

On Thursday night at the UN, the US, Britain and European allies said they would draft a security council sanctions resolution on Syria. "The Syrian government has not changed course," Britain's deputy UN ambassador, Philip Parham, told reporters after a closed-door council meeting on Syria. "In fact, if anything, its actions over the last two weeks have escalated."

"The time has come for the council to take further actions to step up the pressure against those who are responsible for the violence against the citizens of Syria." The EU foreign policy chief, Lady Ashton, said there had been a "complete loss of Bashar Assad's legitimacy in the eyes of the Syrian people".

One veteran dissident in Damascus said: "I am jubilant. This came at the right time for the street." He said protesters were telling him they wanted to dance in the streets. A middle aged woman in Homs said: "More protesters will go out now."

Razan Zeitouneh, a lawyer in Damascus, said: "This is the right thing to happen after five months of killing civilians. The international community must take its role towards the Syrian regime more seriously and this statement is the right start. I hope to see a more collective role now, which means the UN security council, and I hope to see the Syrian file referred to the international criminal court soon." A computer expert in his 20s from Hama said: "We still want this to come from all the other countries, too, and for ambassadors to be withdrawn and Syrian diplomatic staff to be kicked out."

To put pressure on Assad, Obama said the US was stepping up sanctions, including freezing Syria's assets and banning petroleum products of Syrian origin. But he insisted "the US cannot and will not impose this transition upon Syria".

"It is up to the Syrian people to choose their own leaders and we have heard their strong desire that there not be foreign intervention in their movement," Obama said.

On Thursday night Syria's ambassador to the UN Bashar Ja'afari said the US was waging a "diplomatic and humanitarian war" against his country along with some other UN security council members.

"What the US will support is an effort to bring about a Syria that is democratic, just and inclusive for all Syrians. We will support this outcome by pressuring President Assad to get out of the way of this transition and standing up for the universal rights of the Syrian people along with others in the international community."

The US has calibrated its response to the violence in Syria, wary of Damascus's strategic role in the Arab world and the risk that the crisis could be exported beyond its borders.

Washington has been cautious about putting its authority on the line, fearing damage to its standing if Assad were to defy its calls for him to go.

The call from western capitals came as it was revealed that UN human rights investigators have listed the names of 50 regime figures who could be prosecuted by the international criminal court (ICC) for crimes committed against civilians during the violent crackdown on pro-democracy demonstrators. The list is believed to include officials from the president's inner circle and security agencies. It marks the first time that government insiders have faced the prospect of criminal charges since the five-month uprising began.

A decision on whether to refer the names to the ICC is likely to be made on Thursday.

The UN report accuses officials of torture, summary executions and abuse of children – allegations that could amount to crimes against humanity. It accuses security forces of indiscriminately firing at demonstrators, sometimes from helicopters, and says injured protesters have been killed inside hospitals, even being locked alive in mortuary freezers.

It says Syrian officials confirmed that about 1,900 demonstrators had been killed by mid-July, and states that hundreds more have been killed since.

"Children have not only been targeted by security forces but they have been repeatedly subject to the same human rights and criminal violations as adults, including torture," the report said.

The authors were denied access to Syria and spent four months interviewing defectors and demonstrators who had fled the country.

Dozens of former members of the security forces made their way to Amman and Istanbul, where they have detailed the orders given to them by senior officers to attack demonstrators.

Activists and defectors have compiled details of alleged atrocities by troops whose commanders insist they are targeting terrorists holding their communities to ransom.

The communities have regularly given a diametrically opposed version of events, claiming that the armed men terrorising them are government-backed militias, known as al-shabiha or ghosts, who work with security forces.

One defector, a conscript who was deployed to the southern city of Dera'a in April, said his unit's first order was not to shoot at armed men. "The officer said they were with us," the soldier said. "They said only to shoot at the demonstrators."

In a telephone conversation on Wednesday with UN secretary general Ban Ki-moon, Assad said the operations in the restive Syrian cities of Latakia and Homs had finished. However, activists on the ground reported on Wednesday that security forces were still active in both places. In Latakia, a city that has been the subject of a four-day military assault, security centres were overflowing with detainees and prisoners were being held in the city's main football stadium and a cinema.

The push into Latakia ordered by commanders this week was stridently criticised by other nations in the region, with Saudi Arabia, Bahrain, Tunisia and Qatar withdrawing their ambassadors and Turkey warning it had uttered its "last words" on the crackdown.